Conventionally, the rotation of electric appliances that use a motor as a driving force, and, in particular, the rotation of appliances such as home-use food crushing mixers, have been able to be mechanically overloaded depending on the kind and amount of food put in the appliance, and a greater load could stop the rotation of the food crushing mixers.
Such a great mechanical load increases a current flowing through a current circuit that rotates the motor. In addition, an excessive load decreases the rotation speed of the motor remarkably or stops the rotation of the motor. Such a remarkable decease in the motor rotation speed or the stopping of the motor increases the current, thereby damaging the motor due to, for example, a short circuit of a coil of the motor or burning and cutting of the coil.
A protector is known that senses excessive heat produced by a motor or an excessively increased current flowing through a motor driving circuit and that immediately interrupts the current flowing through the motor driving circuit in order to prevent the aforementioned malfunction and protect the motor.
As such a protector, a motor protecting apparatus has been proposed that includes a built-in exothermic resistor in series with a contact circuit, that operates a bimetallic element in response to heat production from the resistor caused by an excessive current, and that opens the contact circuit so as to interrupt the current (e.g., patent documents 1 to 3).
In the meantime, to interrupt a circuit through which an overload current is flowing, a motor protecting apparatus understandably needs to start an interrupting operation in a short time; in addition, once the interrupting operation starts, it is required for safety that the conductive state not be achieved again until the power is turned off after the fault is eliminated, i.e., until a certain operation is performed by a person.
However, in a resistor arranged in series with a contact circuit, repetitive operations of energization and interruption occur due to an automatic restoration resulting from a temperature decrease after current interruption. Although the motor may be prevented from being suddenly damaged, the repetitive operations of energization and interruption has a defect in, for example, that it is unclear whether or not the appliance is being operated.
To prevent such repetitive operations, a protector is proposed that includes a built-in positive-characteristic thermal resistor in parallel with a contact circuit, wherein the protector self-holds an interrupting-operation state by maintaining a current interrupting operation with a bimetallic element by making use of the heat produced by the resistor (see, for example, patent document 4).